Introduce RFC mdBook web page (#60)

- Closes https://github.com/polkadot-fellows/RFCs/issues/53
- Preview available [here](https://paritytech.github.io/RFCs/).

![Screenshot 2023-12-19 at 13 55
05](https://github.com/polkadot-fellows/RFCs/assets/12039224/ee06a7ad-231e-4077-a773-c6319661dfb6)


---

### What it does

- The workflow gathers the source markdown files of RFCs - approved
ones, and proposed ones (from open PRs).
- The proposed RFCs are separated into new (<7 days since PR created)
and stale (>30 days since PR updated).
- The RFCs are extended with a table of contents, and a link to the
source.
- The RFCs are build into a web page using
[mdBook](https://github.com/rust-lang/mdBook).
- The built web page gets pushed to `gh-pages` branch - to be published
in Github Pages.
- The workflow is triggered on merge to `main`, and periodically once a
day.

.github/scripts/build-mdbook-summary.js

@@ -0,0 +1,38 @@
+/**
+ * mdBook relies on the creation of a special SUMMARY.md file
+ * https://rust-lang.github.io/mdBook/format/summary.html
+ * This script constructs the summary out of the available source files.
+ */
+
+const fs = require('fs');
+const summaryPath = "mdbook/src/SUMMARY.md"
+
+module.exports = async ({github, context}) => {
+  fs.writeFileSync(summaryPath, "# Summary\n\n[Introduction](introduction.md)\n") // Starting point.
+
+  const appendRfcsToSummary = (dirPath) => {
+    for (const filename of fs.readdirSync(dirPath)) {
+      if (!filename.endsWith(".md")) continue;
+      const filePath = dirPath + filename
+      const text = fs.readFileSync(filePath)
+      const title = text.toString().split(/\n/)
+        .find(line => line.startsWith("# ") || line.startsWith(" # "))
+        .replace("# ", "")
+      // Relative path, without the src prefix (format required by mdbook)
+      const relativePath = filePath.replace("mdbook/src/", "")
+      fs.appendFileSync(summaryPath, `- [${title}](${relativePath})\n`)
+    }
+  }
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Approved\n\n")
+  appendRfcsToSummary("mdbook/src/approved/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Newly Proposed\n\n")
+  appendRfcsToSummary("mdbook/src/new/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Proposed\n\n")
+  appendRfcsToSummary("mdbook/src/proposed/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Stale\n\n")
+  appendRfcsToSummary("mdbook/src/stale/")
+}

.github/scripts/gather-mdbook-sources.js

@@ -0,0 +1,74 @@
+/**
+ * This scripts gathers source markdown files of approved and proposed RFCS into the mdbook/src directory.
+ */
+
+const fs = require('fs');
+
+// The amount of days that an RFC is considered "new".
+// Counted from the creation of a given PR.
+const NEW_RFC_PERIOD_DAYS = 7
+
+// The amount of days that an RFC is considered "stale".
+// Counted from the last update on a PR.
+const STALE_RFC_PERIOD_DAYS = 30
+
+const dateDaysBefore = (daysBefore) => {
+  const result = new Date()
+  result.setDate(result.getDate() - daysBefore)
+  return result
+}
+
+[
+  "mdbook/src/approved",
+  "mdbook/src/new",
+  "mdbook/src/stale",
+  "mdbook/src/proposed"
+].forEach(path => fs.mkdirSync(path, {resursive: true}))
+
+const TOC = "**Table of Contents**\n\n<\!-- toc -->\n"
+
+module.exports = async ({github, context}) => {
+  const owner = 'polkadot-fellows'
+  const repo = 'RFCs'
+  const prs = await github.paginate(github.rest.pulls.list, {owner, repo, state: 'open'})
+
+  /*
+    The open PRs are potential proposed RFCs.
+    We iterate over them and filter those that include a new RFC markdown file.
+  */
+  for (const pr of prs) {
+    const addedMarkdownFiles = (
+      await github.rest.pulls.listFiles({
+        owner, repo,
+        pull_number: pr.number,
+      })
+    ).data.filter(
+      (file) => file.status === "added" && file.filename.startsWith("text/") && file.filename.includes(".md"),
+    );
+    if (addedMarkdownFiles.length !== 1) continue;
+    const [rfcFile] = addedMarkdownFiles;
+    const rawText = await (await fetch(rfcFile.raw_url)).text();
+
+    const isNew = new Date(pr.created_at) > dateDaysBefore(NEW_RFC_PERIOD_DAYS)
+    const isStale = new Date(pr.updated_at) < dateDaysBefore(STALE_RFC_PERIOD_DAYS)
+    const status = isNew ? 'new' : (isStale ? 'stale' : 'proposed')
+
+    const filename = rfcFile.filename.replace("text/", "")
+
+    fs.writeFileSync(
+      `mdbook/src/${status}/${filename}`,
+      `[(source)](${pr.html_url})\n\n`
+      + TOC
+      + rawText
+    )
+  }
+
+  // Copy the approved (already-merged) RFCs markdown files, first adding a source link at the top and a TOC.
+  for (const file of fs.readdirSync("text/")) {
+    if (!file.endsWith(".md")) continue;
+    const text = `[(source)](https://github.com/polkadot-fellows/RFCs/blob/main/text/${file})\n\n`
+      + TOC
+      + fs.readFileSync(`text/${file}`)
+    fs.writeFileSync(`mdbook/src/approved/${file}`, text)
+  }
+}